Radiation Physics for Medical Physiscists - E.B. Podgorsak

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Mansfield’s doctoral thesis was on the physics of nuclear magnetic resonance (NMR), at the time used for studies of chemical structure, and he spent the 1960s perfecting his understanding of NMR techniques. In the early 1970s Mansfield began studies in the use of NMR for imaging and developed magnetic field gradient techniques for producing two-dimensional images in arbitrary planes through a human body. The term “nuclear” was dropped from NMR imaging and the technique is now referred to as magnetic resonance imaging or MRI. Mansfield is also credited with developing the MRI protocol called the “echo planar imaging” which in comparison to standard techniques allows a much faster acquisition of images and makes functional MRI (fMRI) possible.

Mansfield shared the 2003 Nobel Prize in Physiology or Medicine with

Paul C. Lauterbur “for their discoveries concerning magnetic resonance imaging”.

MARSDEN, Ernest (1889–1970)

New Zealand-born physicist who made a remarkable contribution to science in New Zealand and England. He studied physics at the University of Manchester and as a student of Ernest Rutherford, in collaboration with Hans Geiger, carried out the α-particle scattering experiments that inspired Rutherford to propose the atomic model, currently known as the RutherfordBohr model of the atom. In 1914 he returned to New Zealand to become Professor of Physics at Victoria University in Wellington. In addition to scientific work, he became involved with public service and helped in setting up the New Zealand Department of Scientific and Industrial Research. During World War II, he became involved with radar technology in defense work and in 1947 he was elected president of the Royal Society of New Zealand. He then returned to London as New Zealand’s scientific liaison o cer and “ambassador” for New Zealand science. In 1954 he retired to New Zealand and remained active on various advisory committees as well as in radiation research until his death in 1970.

MEITNER, Lise (1878–1968)

Austrian-born physicist who studied physics at the University of Vienna and was strongly influenced in her vision of physics by Ludwig Boltzmann, a leading theoretical physicist of the time. In 1907 Meitner moved to Berlin to work with Max Planck and at the University of Berlin she started a life-long friendship and professional association with radiochemist Otto Hahn. At the Berlin University both Meitner and Hahn were appointed as scientific associates and progressed through academic ranks to attain positions of professor.

During her early days in Berlin, Meitner discovered the element protactinium with atomic number Z = 91 and also discovered, two years before Auger, the non-radiative atomic transitions that are now referred to as the Auger e ect. Meitner became the first female physics professor in Germany

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but, despite her reputation as an excellent physicist, she, like many other Jewish scientists, had to leave Germany during the 1930s. She moved to Stockholm and left behind in Berlin her long-term collaborator and friend Otto Hahn, who at that time was working with Fritz Strassmann, an analytical chemist, on studies of uranium bombardment with neutrons. Their experiments, similarly to those reported by Irene Joliot-Curie and Pavle Savi´c were yielding surprising results suggesting that in neutron bombardment uranium was splitting into smaller atoms with atomic masses approximately half of that of uranium. In a letter Hahn described the uranium disintegration by neutron bombardment to Meitner in Stockholm and she, in collaboration with Otto Frisch, succeeded in providing a theoretical explanation for the uranium splitting and coined the term nuclear fission to name the process.

The 1944 Nobel Prize in Chemistry was awarded to Hahn “for the discovery of the nuclear fission”. The Nobel Committee unfortunately ignored the contributions by Strassmann, Meitner and Frisch to the theoretical understanding of the nuclear fission process. Most texts dealing with the history of nuclear fission now recognize the four scientists: Hahn, Strassmann, Meitner, and Frisch as the discoverers of the fission process.

Despite several problems that occurred with recognizing Meitner’s contributions to modern physics, her scientific work certainly was appreciated and is given the same ranking in importance as that of Marie Curie. In 1966 Meitner together with Hahn and Strassmann shared the prestigious Enrico Fermi Award. In honor of Meitner’s contributions to modern physics the element with atomic number 109 was named meitnerium (Mt).

MENDELEYEV, Dmitri Ivanoviˇc (1834–1907)

Russian physical chemist, educated at the University of St. Petersburg where he obtained his M.A. in Chemistry in 1856 and doctorate in Chemistry in 1865. The years between 1859 and 1861 Mendeleyev spent studying in Paris and Heidelberg. He worked as Professor of Chemistry at the Technical Institute of St. Petersburg and the University of St. Petersburg from 1862 until 1890 when he retired from his academic posts for political reasons. From 1893 until his death in 1907 he was Director of the Bureau of Weights and Measures in St. Petersburg.

While Mendeleyev made contributions in many areas of general chemistry as well as physical chemistry and was an excellent teacher, he is best known for his 1869 discovery of the Periodic Law and the development of the Periodic Table of Elements. Until his time elements were distinguished from one another by only one basic characteristic, the atomic mass, as proposed by John Dalton in 1805. By arranging the 63 then-known elements by atomic mass as well as similarities in their chemical properties, Mendeleyev obtained a table consisting of horizontal rows or periods and vertical columns or groups. He noticed several gaps in his Table of Elements and predicted that they represented elements not yet discovered. Shortly afterwards ele-

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ments gallium, germanium and scandium were discovered filling three gaps in the table, thereby confirming the validity of Mendeleyev’s Periodic Table of Elements. Mendeleyev’s table of more than a century ago is very similar to the modern 21st century Periodic Table, except that the 111 elements of the modern periodic table are arranged according to their atomic number Z in contrast to Mendeleyev’s table in which the 63 known elements were organized according to atomic mass. To honor Mendeleyev’s work the element with atomic number Z of 101 is called mendelevium.

MILLIKAN, Robert Andrews (1868–1952)

American physicist, educated at Oberlin College (Ohio) and Columbia University in New York where he received a doctorate in Physics in 1895. He then spent a year at the Universities of Berlin and G¨otingen, before accepting a position at the University of Chicago in 1896. By 1910 he was Professor of Physics and remained in Chicago until 1921 when he was appointed Director of the Norman Bridge Laboratory of Physics at the California Institute of Technology (Caltech) in Pasadena. He retired in 1946.

Millikan was a gifted teacher and experimental physicist. During his early years at Chicago he authored and coauthored many physics textbooks to help and simplify the teaching of physics. As a scientist he made many important discoveries in electricity, optics and molecular physics. His earliest and best known success was the accurate determination, in 1910, of the electron charge with the “falling-drop method” now commonly referred to as the Millikan experiment. He also verified experimentally the Einstein’s photoelectric e ect equation and made the first direct photoelectric determination of Planck’s quantum constant h.

The 1923 Nobel Prize in Physics was awarded to Millikan “for his work on the elementary charge of electricity and on the photoelectric e ect”.

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MOSSBAUER, Rudolf Ludwig (born in 1929)

German physicist, educated at the Technische Hochschule (Technical University) in Munich, where he received his doctorate in Physics in 1958, after carrying out the experimental portion of his thesis work in Heidelberg at the Institute for Physics of the Max Planck Institute for Medical Research. During 1959 M¨ossbauer worked as scientific assistant at the Technical University in Munich and from 1960 until 1962 as Professor of Physics at the California Institute of Technology (Caltech) in Pasadena. In 1962 he returned to the Technical Institute in Munich as Professor of Experimental Physics and stayed there his whole professional career except for the period 1972-1977 which he spent in Grenoble as the Director of the Max von Laue Institute.

M¨ossbauer is best known for his 1957 discovery of recoil-free gamma ray resonance absorption; a nuclear e ect that is named after him and was used to verify Albert Einstein’s theory of relativity and to measure the magnetic field of atomic nuclei. The M¨ossbauer e ect involves the emission and absorp-

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tion of gamma rays by atomic nuclei. When a free excited nucleus emits a gamma photon, the nucleus recoils in order to conserve momentum. The nuclear recoil uses up a minute portion of the decay energy, so that the shift in the emitted photon energy prevents the absorption of the photon by another target nucleus of the same species. While working on his doctorate thesis in Heidelberg, M¨ossbauer discovered that, by fixing emitting and absorbing nuclei into a crystal lattice, the whole lattice gets involved in the recoil process, minimizing the recoil energy loss and creating an overlap between emission and absorption lines thereby enabling the resonant photon absorption process and creating an extremely sensitive detector of photon energy shifts.

M¨ossbauer received many awards and honorable degrees for his discovery; most notably, he shared with Robert Hofstadter the 1961 Nobel Prize in Physics “for his researches concerning the resonance absorption of gamma radiation and his discovery in this connection of the e ect which bears his name.” Hofstadter received his share of the 1961 Nobel Prize for his pioneering studies of electron scattering in atomic nuclei.

MOSELEY, Henry Gwen Je reys (1887–1915)

British physicist, educated at the University of Oxford where he graduated in 1910. He began his professional career at the University of Manchester as Lecturer in physics and research assistant under Ernest Rutherford.

Based on work by Charles Barkla who discovered characteristic x rays and on work of the team of William Bragg and Lawrence Bragg who studied x ray di raction, Moseley undertook in 1913 a study of the K and L characteristic x rays emitted by then-known elements from aluminum to gold. He found that the square root of the frequencies of the emitted characteristic x-ray lines plotted against a suitably chosen integer Z yielded straight lines. Z was subsequently identified as the number of positive charges (protons) and the number of electrons in an atom and is now referred to as the atomic number Z. Moseley noticed gaps in his plots that corresponded to atomic numbers Z of 43, 61, and 75. The elements with Z = 43 (technetium) and Z = 61 (promethium) do not occur naturally but were produced artificially years later. The Z = 75 element (rhenium) is rare and was discovered only in 1925. Moseley thus found that the atomic number of an element can be deduced from the element’s characteristic spectrum (non-destructive testing). He also established that the periodic table of elements should be arranged according to the atomic number Z rather than according to the atomic mass number A as was common at his time.

There is no question that Moseley during a short time of two years produced scientific results that were very important for the development of atomic and quantum physics and were clearly on the level worthy of Nobel Prize. Unfortunately, he perished during World War I shortly after starting his professional career in physics.

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NISHINA, Yoshio (1890–1951)

Japanese physicist, educated at the University of Tokyo where he graduated in 1918. He worked three years as an assistant at the University of Tokyo and then spent several years in Europe: 1921-1923 at the University of Cambridge with Ernest Rutherford and 1923–1928 at the University of Copenhagen with Niels Bohr. From 1928 to 1948 he worked at the University of Tokyo.

Nishina is best known internationally for his collaboration with Oskar Klein on the cross section for Compton scattering in 1928 (Klein-Nishina formula). Upon return to Japan from Europe, Nishina introduced the study of nuclear and high energy physics in Japan and trained many young Japanese physicists in the nuclear field. During World War II Nishina was the central figure in the Japanese atomic weapons program that was competing with the American Manhattan project and using the same thermal uranium enrichment technique as the Americans. The race was tight; however, the compartmentalization of the Japanese nuclear weapons program over competing ambitions of the army, air force and the navy gave the Americans a definite advantage and eventual win in the nuclear weapons competition that resulted in the atomic bombs over Hiroshima and Nagasaki and Japanese immediate surrender in 1945.

PAULI, Wolfgang (1900–1958)

Austrian-born physicist, educated at the University of Munich where he obtained his doctorate in Physics in 1921. He spent one year at the University of G¨ottingen and one year at the University of Copenhagen before accepting a Lecturer position at the University of Hamburg (1923-1928). From 1928 to 1958 he held an appointment of Professor of Theoretical Physics at the Eidgen¨ossische Technische Hochschule in Z¨urich. From 1940 to 1946 Pauli was a visiting professor at the Institute for Advanced Study in Princeton.

Pauli is known as an extremely gifted physicist of his time. He is best remembered for enunciating the existence of the neutrino in 1930 and for introducing the exclusion principle to govern the states of atomic electrons in general. The exclusion principle is now known as the Pauli Principle and contains three components. The first component states that no two electrons can be at the same place at the same time. The second component states that atomic electrons are characterized by four quantum numbers: principal, orbital, magnetic and spin. The third component states that no two electrons in an atom can occupy a state that is described by exactly the same set of the four quantum numbers. The exclusion principle was subsequently expanded to other electronic and fermionic systems, such as molecules and solids.

The 1945 Nobel Prize in Physics was awarded to Pauli “for his discovery of the Exclusion Principle, also called the Pauli Principle”.

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PLANCK, Max Karl Ernst (1858–1947)

German physicist, educated at the University of Berlin and University of Munich where he received his doctorate in Physics in 1879. He worked as Assistant Professor at the University of Munich from 1880 until 1885, then Associate Professor at the University of Kiel until 1889 and Professor of Physics at the University of Berlin until his retirement in 1926.

Most of Planck’s work was on the subject of thermodynamics in general and studies of entropy and second law of thermodynamics in particular. He was keenly interested in the blackbody problem and the inability of classical mechanics to predict the blackbody spectral distribution. Planck studied the blackbody spectrum in depth and concluded that it must be electromagnetic in nature. In contrast to classical equations that were formulated for blackbody radiation by Wien and Rayleigh, with Wien’s equation working only at high frequencies and Rayleigh’s working only at low frequencies, Planck formulated an equation that predicted accurately the whole range of applicable frequencies and is now known as Planck’s equation for blackbody radiation. The derivation was based on the revolutionary idea that the energy emitted by a resonator can only take on discrete values or quanta, with the quantum energy ε equal to hν, where ν is the frequency and h a universal constant now referred to as the Planck’s constant. Planck’s idea of quantization has been successfully applied to the photoelectric e ect by Albert Einstein and to the atomic model by Niels Bohr.

In 1918 Planck was awarded the Nobel Prize in Physics “in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta”. In addition to Planck’s constant and Planck’s formula, Planck’s name and work are honored with the Max Planck Medal that is awarded annually as the highest distinction by the German Physical Society (Deutsche Physikalische Gesellschaft) and the Max Planck Society for the Advancement of Science that supports basic research at 80 research institutes focusing on research in biology, medicine, chemistry, physics, technology and humanities.

PURCELL, Edward Mills (1912–1997)

American physicist, educated at Purdue University in Indiana where he received his Bachelor’s degree in Electrical Engineering in 1933 and Harvard where he received his doctorate in Physics in 1938. After serving for two years as Lecturer of physics at Harvard, he worked at the Massachusetts Institute of Technology on development of new microwave techniques. In 1945 Purcell returned to Harvard as Associate Professor of Physics and became Professor of Physics in 1949.

Purcell is best known for his 1946 discovery of nuclear magnetic resonance (NMR) with his students Robert Pound and Henry C. Torrey. NMR o ers an elegant and precise way of determining chemical structure and properties of materials and is widely used not only in physics and chemistry but also in

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medicine where, through the method of magnetic resonance imaging (MRI), it provides non-invasive means to image internal organs and tissues of patients.

In 1952 Purcell shared the Nobel Prize in Physics with Felix Bloch “for their development of new methods for nuclear magnetic precision measurements and discoveries in connection therewith”.

RAYLEIGH, John William Strutt (1842–1919)

English mathematician and physicist who studied mathematics at the Trinity College in Cambridge. Being from an a uent family he set up his physics laboratory at home and made many contributions to applied mathematics and physics from his home laboratory. From 1879 to 1884 Rayleigh was Professor of Experimental Physics and Head of the Cavendish Laboratory at Cambridge, succeeding James Clark Maxwell. From 1887 to 1905 he was Professor of Natural Philosophy at the Royal Institution in London.

Rayleigh was a gifted researcher and made important contributions to all branches of physics known at his time, having worked in optics, acoustics, mechanics, thermodynamics, and electromagnetism. He is best known for explaining that the blue color of the sky arises from the scattering of light by dust particles in air and for relating the degree of light scattering to the wavelength of light (Rayleigh scattering). He also accurately defined the resolving power of a di raction grating; established standards of electrical resistance, current, and electromotive force; discovered argon; and derived an equation describing the distribution of wavelengths in blackbody radiation (the equation applied only in the limit of large wavelengths).

In 1904 Rayleigh was awarded the Nobel Prize in Physics “for his investigations of the densities of the most important gases and for his discovery of the noble gas argon in connection with these studies”. He discovered argon together with William Ramsey who obtained the 1904 Nobel Prize in Chemistry for his contribution to the discovery.

RICHARDSON, Owen Willans (1879–1959)

British physicist, educated at Trinity College in Cambridge from where he graduated in 1990 as a student of Joseph J. Thomson at the Cavendish Laboratory. He was appointed Professor of Physics at Princeton University in the United States in 1906 but in 1914 returned to England to become Professor of Physics at King’s College of the University of London.

Richardson is best known for his work on thermionic emission of electrons from hot metallic objects that enabled the development of radio and television tubes as well as modern x-ray (Coolidge) tubes. He discovered the equation that relates the rate of electron emission to the absolute temperature of the metal. The equation is now referred to as the Richardson’s law or the Richardson-Dushman equation.

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In 1928 Richardson was awarded the Nobel Prize in Physics “for his work on the thermionic phenomenon and especially for the law that is named after him”.

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RONTGEN, Wilhelm Conrad (1845–1923)

German physicist, educated at the University of Utrecht in Holland and University of Z¨urich where he obtained his doctorate in Physics in 1869. He worked as academic physicist at several German universities before accepting a position of Chair of Physics at the University of Giessen in 1979. From 1888 until 1900 he was Chair of Physics at the University of W¨urzburg and from 1900 until 1920 he was Chair of Physics at the University of Munich.

R¨ontgen was active in many areas of thermodynamics, mechanics and electricity but his notable research in these areas was eclipsed by his accidental discovery in 1895 of “a new kind of ray”. The discovery occurred when R¨ontgen was studying cathode rays (now known as electrons, following the work of Joseph J. Thomson) in a Crookes tube, a fairly mundane and common experiment in physics departments at the end of the 19th century. He noticed that, when his energized Crookes tube was enclosed in a sealed black and light-tight envelope, a paper plate covered with barium platinocianide, a known fluorescent material, became fluorescent despite being far removed from the discharge tube. R¨ontgen concluded that he discovered an unknown type of radiation, much more penetrating than visible light and produced when cathode rays strike a material object inside the Crookes tube. He named the new radiation x rays and the term is generaly used around the World. However, in certain countries x rays are often called R¨ontgen rays. In 1912 Max von Laue showed with his crystal di raction experiments that x rays are electromagnetic radiation similar to visible light but of much smaller wavelength. In tribute to R¨ontgen’s contributions to modern physics the element with the atomic number 111 was named r¨ontgenium (Rg).

In 1901 the first Nobel Prize in Physics was awarded to R¨ontgen “in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays subsequently named after him”.

RUTHERFORD, Ernest (1871–1937)

New Zealand-born nuclear physicist, educated at the Canterbury College in Christchurch, New Zealand (B.Sc. in Mathematics and Physical Science in 1894) and at the Cavendish Laboratory of the Trinity College in Cambridge. He received his science doctorate from the University of New Zealand in 1901. Rutherford was one of the most illustrious physicists of all time and his professional career consists of three distinct periods: as MacDonald Professor of Physics at McGill University in Montreal (1898–1907); as Langworthy Professor of Physics at the University of Manchester (1908–1919); and as Cavendish Professor of Physics at the Cavendish Laboratory of Trinity College in Cambridge (1919–1937).

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With the exception of his early work on magnetic properties of iron exposed to high frequency oscillations, Rutherford’s career was intimately involved with the advent and growth of nuclear physics. Nature provided Rutherford with α particles, an important tool for probing the atom, and he used the tool in most of his exciting discoveries that revolutionized physics in particular and science in general.

Before moving to McGill in 1898, Rutherford worked with Joseph J. Thomson at the Cavendish Laboratory on detection of the just-discovered x rays (Wilhelm R¨ontgen in 1895) through studies of electrical conduction of gases caused by x-ray ionization of air. He then turned his attention to the just-discovered radiation emanating from uranium (Henri Becquerel in 1896) and radium (Pierre Curie and Marie Curie in 1898) and established that uranium radiation consists of at least two components, each of particulate nature but with di erent penetrating powers. He coined the names α and β particles for the two components.

During his 10 years at McGill, Rutherford published 80 research papers, many of them in collaboration with Frederick Soddy, a chemist who came to McGill from Oxford in 1900. Rutherford discovered the radon gas as well as gamma rays and speculated that the gamma rays were similar in nature to x rays. In collaboration with Soddy he described the transmutation of radioactive elements as a spontaneous disintegration of atoms and defined the half-life of a radioactive substance as the time it takes for its activity to drop to half of its original value. He noted that all atomic disintegrations were characterized by emissions of one or more of three kinds of rays: α, β, and γ.

During the Manchester period Rutherford determined that α particles were helium ions. He guided Hans Geiger and Ernest Marsden through the now-famous α particle scattering experiment and, based on the experimental results, in 1911 proposed a revolutionary model of the atom which was known to have a size of the order of 10−10 m. He proposed that most of the atomic mass is concentrated in a miniscule nucleus with a size of the order of 10−15 m and that the atomic electrons are distributed in a cloud around the nucleus. In 1913 Niels Bohr expanded Rutherford’s nuclear atomic model by introducing the idea of the quantization of electrons’ angular momenta and the resulting model is now called the Rutherford-Bohr atomic model. During his last year at Manchester, Rutherford discovered that nuclei of nitrogen, when bombarded with α particles, artificially disintegrate and produce protons in the process. Rutherford was thus first in achieving artificial transmutation of an element through a nuclear reaction.

During the Cambridge period Rutherford collaborated with many worldrenowned physicists such as John Cocroft and Ernest Walton in designing a proton accelerator now called the Cocroft-Walton machine, and with James Chadwick in discovering the neutron in 1932. Rutherford’s contributions to modern physics are honored with the element of atomic number 104 which was named rutherfordium (Rf).

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In 1908 Rutherford was awarded the Nobel Prize in Chemistry “for his investigations into the disintegration of the elements and the chemistry of radioactive substances”.

RYDBERG, Johannes (1854–1919)

Swedish physicist, educated at Lund University. He obtained his Ph.D. in Mathematics in 1879 but worked all his professional life as a physicist at Lund University where he became Professor of Physics and Chairman of the Physics department.

Rydberg is best known for his discovery of a mathematical expression that gives the wavenumbers of spectral lines for various elements and includes a constant that is now referred to as the Rydberg constant (R∞ = 109 737 cm−1). In honor of Rydberg’s work in physics the absolute value of the ground state energy of the hydrogen atom is referred to as the Rydberg energy (ER = 13.61 eV).

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SCHRODINGER, Erwin (1887–1961)

Austrian physicist, educated at the University of Vienna where he received his doctorate in Physics in 1910. He served in the military during World War I and after the war moved through several short-term academic positions until in 1921 he accepted a Chair in Theoretical Physics at the University of Z¨urich. In 1927 he moved to the University of Berlin as Planck’s successor. The rise of Hitler in 1933 convinced Schr¨odinger to leave Germany. After spending a year at Princeton University, he accepted a post at the University of Graz in his native Austria. The German annexation of Austria in 1938 forced him to move again, this time to the Institute for Advanced Studies in Dublin where he stayed until his retirement in 1955.

Schr¨odinger made many contributions to several areas of theoretical physics; however, he is best known for introducing wave mechanics into quantum mechanics. Quantum mechanics deals with motion and interactions of particles on an atomic scale and its main attribute is that it accounts for the discreteness (quantization) of physical quantities in contrast to classical mechanics in which physical quantities are assumed continuous. Examples of quantization were introduced by Max Planck who in 1900 postulated that oscillators in his blackbody emission theory can possess only certain quantized energies; Albert Einstein who in 1905 postulated that electromagnetic radiation exists only in discrete packets called photons; and Niels Bohr who in 1913 introduced the quantization of angular momenta of atomic orbital electrons. In addition, Louis de Broglie in 1924 introduced the concept of wave-particle duality.

Schr¨odinger’s wave mechanics is based on the so-called Schr¨odinger’s wave equation, a partial di erential equation that describes the evolution over time of the wave function of a physical system. Schr¨odinger and other physicists have shown that many quantum mechanical problems can be solved by means